With the coming of the information society and the personal communications era, the demand on wireless communications technology is growing rapidly, but the frequency resources are very limited. A code division multiple access ("CDMA") technique is the only efficient way to resolve the contradiction between limited frequency resources and demand for high capacity. The capacity of traditional wireless multiple access techniques, e.g., frequency division multiple access ("FDMA") and time division multiple access ("TDMA"), is fixed once designed, i.e., additional users can not be introduced beyond that capacity limit. But CDMA is different in that the capacity is only limited by the interference level and thus results in the advantages of large capacity and soft capacity. That is, introducing an additional user is not precluded even though it may lead to reduced signal-to-noise ratio and quality of communications. So, unlike FDMA or TDMA, an insurmountable capacity limit does not exist.
As is noted above, the capacity of a CDMA system is interference-limited, thus, whether the interference level can be controlled or not determines the system's quality. Generally, the interference in the system consists of four parts: the first is local noise, which may be reduced by applying a low noise amplifier; the second is multiple access interference ("MAI"), which comes from the other users in the system; the third is inter-code or inter-symbol interference ("ISI"); and the fourth is neighboring cell or adjacent channel interference ("ACI"). By employing well-designed multiple access codes, MAI, ISI and ACI can be reduced or even eliminated.
In any CDMA system, each user has a specific spread spectrum multiple access code for identification. Furthermore, to reduce the users' mutual interference, the spread spectrum multiple access codes must be orthogonal to each other. Indeed, orthogonality between any two users' signals is always required in any multiple access system. Given that the channel is an ideal linear time-invariant system, and accurate synchronization is realized in the system, then orthogonality between any two users' signals can be achieved. Unfortunately, there is no such ideal channel in practice. Besides, it is quite difficult to maintain strict synchronization. That is why it is important to employ a good multiple access technique. As for a CDMA technique, well designed multiple access codes are the root of the system.
It is known that the wireless channel is a typical random time-varying channel, in which there exists not only random frequency dispersion (Doppler frequency shift) but also random time dispersion (multi-path propagation). The former introduces time selective fading to the received signals, i.e., the received signal's frequency varies randomly with time. The latter introduces frequency selective fading to the received signals, i.e. different frequency spectrum components of the received signal vary differently with time. The fading deteriorates the system's performance seriously and at the same time, reduces the system's capacity. This is especially true for the channel's time dispersion, which is caused by multi-path propagation: it prevents signals from arriving simultaneously, so ISI and MAI are caused and the system's capacity is drastically reduced. When the relative time delay between signals is zero, it is quite easy to achieve orthogonality between signals, indeed any orthogonal codes can meet that requirement, but when the relative delay between signals is non-zero, it becomes very difficult to do so. In fact, it has been proven that there are no such spread spectrum multiple access codes in binary, finite and even complex number spaces. In particular, MAI and ISI contradict one another so that smaller MAI leads to larger ISI and vice versa.
Therefore, the distinction between different CDMA systems lies mainly in the selected multiple access codes, i.e. in a good system, ISI and MAI must both be small, otherwise they must be larger.
Existing CDMA systems have either very low efficiency or have very short communications distance for example about several hundred meters or do nothing to MAI and ISI and then all that can be done is to alleviate them by using relatively good multiple access codes.